Apple 8 manual Digital Conversions, About Bits and Sampling Rates

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Digital Conversions

When an analog audio signal arrives at the inputs of your audio interface, it must be converted into digital information before the computer can deal with it. This process is called analog to digital conversion—handled by the Analog to Digital Converter (ADC) of your audio interface.

At the other end of the signal path, the digital signal needs to be reconverted into an analog signal, so that it can be heard through analog audio playback systems—an amplifier and speakers. This process is called digital to analog conversion, which is performed by the Digital to Analog Converter (DAC) of your audio interface.

About Bits and Sampling Rates

When recording a sound into your computer, the ADC of your audio interface takes a sample of the sound source, a specific number of times per second. This is known as the sample rate, which is measured in kHz (kilohertz—kilo means thousand) or “x” thousand samples per second. The higher the rate, the more samples the AD converter takes, and the more accurate the digital representation of the sound will be.

Put another way, imagine each sample to be a photograph of a concert that you are trying to describe to a friend. If you had taken one photo every 10 minutes, you’d find it hard to describe the great light show. If you took a thousand photos during that 10 minutes, your friend could easily see what happened. This increased “rate,” or number of photos (samples), delivers a far more accurate overall picture of what happened. This is how sampling works. The more “sonic photos” taken, the more accurately the sound is represented over time. A videotape recording of the concert—running at 25 frames (samples) a second, would give an even more accurate idea of the light show.

Common sample rates used in audio production include: 44.1 kHz or 48 kHz, and high- end systems may use rates from 96 kHz up to 192 kHz.

The sampling resolution—expressed as bit values—determines the precision of the measuring scale used to store (the numbers of) each sample. Remember that computers store audio files as digital information—a series of ones and zeroes. There are three audio file resolutions commonly in use: 8, 16, and 24 bits. 24 bit systems are commonly used these days.

To give you an idea of the relevance of the sampling resolution, imagine two people are building a house. One is using a tape measure marked to the nearest foot. The other has a tape measure marked to the nearest inch. Although the house built with the tape measure accurate to the nearest foot may not fall down, the person using the finer scale will build a more accurate house.

Appendix A Audio and MIDI Basics